Significant new advances were made in discovering mechanisms by which regulatory T cells suppressed anti-viral CD8+ T cell responses in mice chronically infected with Friend retrovirus. A new in vitro assay for detecting Treg-mediated suppression of CD8+ T cell function was developed in which virtually all aspects of in vivo responses are replicated. This system will enable us to directly study intereactions between purified subsets of T cells. Our new information is leading to new avenues for modulating interactions between these critical components of antiviral immunity leading to treatments for chronic retroviral infections. Recent reports indicate that regulatory T cell suppression of CD8+ T cell resopnses is also occurring in HIV infections in humans and the degree of suppression correlates with viral loads. Thus our mouse model is extremely relvant to human retroviral infections and may lead to new therapies to treat chronic HIV infections.